A half-bridge includes two switching elements each having a drive terminal and load path, the load paths of which are connected in series with each other between supply potential terminals during operation, with a supply voltage being applied between the supply potential terminals. An output to which a load can be connected to is arranged between the load paths. By alternatingly switching the two switches on and off form a voltage present between the positive and the negative supply potentials an alternating voltage can be produced that is available at the output of the half-bridge. Driving the two switching elements is performed by two drive signals, one of each being applied to the drive terminal of one of the switching elements. These drive signals can assume an on-level for switching on the individual switching element, and an off-level for switching off the individual switching element.
For monitoring a current flowing through the half-bridge a measuring resistor (shunt resistor) that is connected in series with the load path of one of the switching elements can be used. A measurement voltage that is present across this resistor is a measure of the current flowing through the half-bridge during one half-cycle of the alternating current.
In applications, such as, e.g., lamp ballasts or LLC converters, in which the current flowing through the half-bridge has no direct component the measurement voltage can be used for determining a mean power consumption, because at a constant input voltage the arithmetical mean of this voltage is a measure of the mean power consumption. Calculation of the mean is, e.g., performed using an RC filter.
Half-bridges are, e.g., applied in lamp ballasts. In this case a load connected to the half-bridge includes a series oscillator circuit and a fluorescent lamp (gas discharge lamp). For such kind of load an output current of the half-bridge is an alternating current. Usually switching frequencies, with which the switching elements of a half-bridge in such a lamp
ballast are driven, are in the range of several 10 kHz, such as, for example, 50 kHz; the frequency of the output current and the frequency of the measurement signal, respectively, correspond to the drive frequency in this case.
For an average determination of a measuring signal having a frequency of several 10 kHz an RC filter is necessary having a time constant of about 1 ms. However, for implementing such an RC filter capacitances are required that are too large for implementing them as integrated components in an integrated circuit.
For these and other reasons there is a need for the present invention.